class Solution(object):

def countPrimes(self, n):

"""

:type n: int

:rtype: int

"""

if n < 2:

return 0

A = [0] * (n + 1)

count = 0

for pointer1 in range(2, n):

if A[pointer1] == 0:

count += 1

pointer2 = pointer1

while (pointer2 + pointer1 < n):

pointer2 += pointer1

A[pointer2] = 1

return count